Parkinson disease (PD) is the most common neurodegenerative movement disorder and is characterized by progressive loss of dopamine producing neurons in the substantia nigra pars compacta (SNpc) and intracellular aggregates of the protein alpha-synuclein (?-syn). Increasing evidence points to innate immune system activation as an important mediator of disease. Proinflammatory cytokines TNF-?, IL-1?, IL-6, and IFN- ? are elevated in PD post-mortem brain and CSF, and extensive microgliosis is found around areas of degeneration. GWAS studies implicate polymorphisms in the HLA-DR locus in late-onset PD. Transgenic and viral models of PD in mouse, rat, and non-human-primate recapitulate inflammation and neurodegeneration through microgliosis, cytokine expression, lymphocyte invasion, and gradual loss of TH+ cells in the SNpc. MicroRNAs (miRs) are small noncoding RNAs involved in regulating gene expression via repression of mRNA at the post-transcriptional level. Increasing evidence points to miRs as important regulators of the inflammatory environment. Disrupted miR networks affect inflammation in many CNS disorders, such as multiple sclerosis, amyotrophic lateral sclerosis, and Alzheimer disease. Two miRs with an important role in regulating the inflammatory microenvironment in the CNS are miR-124 and miR-155. miR-124 is responsible for maintaining microglial quiescence through a neuron-to-microglia signaling pathway by targeting CEBP? with subsequent PU.1 pathway down-regulation resulting in decreased expression of CD45, M-CSFR, CD11b, F4/80, and MHCII. In contrast to miR-124, miR-155 sustains increased pro-inflammatory signaling by both promoting release of pro-inflammatory cytokines and downregulating anti-inflammatory signaling pathways. FADD, SOCS-1, IKK, IL13R?1, SMAD-2 and CEBP? are targets of miR-155, which collectively result in IL-1, IL-6, TNF-?, and iNOS upregulation and IL-10, Arg 1, IL-13R, and TGF-?R pathway downregulation. Using an in vivo model in which ?-syn overexpression is produced using adeno-associated virus (AAV), we have observed elevated cytokine expression and IgG deposition at 2 and 4 weeks post transduction with reactive microgliosis at 4 weeks. Subsequent dopaminergic cell loss in the SNpc is observed 6 months post transduction. Using this model, I have obtained data showing an initial reduction of miR-124 expression at 2 weeks post-transduction followed by increased expression at 4 weeks; miR-155 expression is enhanced at 2 weeks. I have also conducted preliminary studies in miR-155 knockout animals; at 4 weeks post-transduction of AAV2-SYN, there was a marked decrease in microgliosis assessed by MHCII staining, compared to WT animals. Our lab has recently shown that MHCII has an important role in ?-syn induced neurodegeneration. In the proposed project, I will examine the role of miR-124 and 155 in ?-syn induced inflammation both in vitro and in vivo. If these miR?s are important in sustaining the inflammatory response, they could be specific, novel biomarkers capable of providing early diagnosis for PD and can be targeted by novel therapeutic treatments.